The field of invention of this valve pertains to valves and more particularly to a valve assembly of the type known as a Kelly foot valve or a mud saver valve used in the rotary system for drilling oil wells.
Conventional drilling of oil wells uses a drill string or sections of drill pipe to pump drilling mud down to a drilling bit at the bottom of the hole being drilled. The drill string also typically is rotated to provide rotary power to the drilling bit at the bottom of the hole. As the hole is progressively drilled deeper and deeper, sections of drill pipe are added to the drill string to allow continued drilling. These sections are typically 30 feet long. The wells are typically from 1000 to 20,000 feet deep. The drill string is supported in the rotary table of the rig and the upper drive section or Kelly is unscrewed. When it is unscrewed, the mud in the Kelly and the hose connecting the top of the Kelly to the other piping on the rig pours out onto the rig floor.
The pouring of the drilling mud onto the rig floor is expensive because of the cost of the mud and is dangerous to the rig crew as it makes the floor slippery.
Prior art valves have been inserted into the drill string at the foot or lower end of the Kelly with different characteristics and with different degrees of success. One solution has been to place a slim O.D. ball valve in the string which is operated manually by the crew.
Other valves have been added which operate automatically based upon bore pressure or upon throttling of the fluid across the valves. These valves and the valve of the present invention are typically installed in a sub called a Kelly Saver. The term Kelly Saver comes from the fact that the section of square pipe at the top of the drill pipe which is engaged by the rotary table to turn the drill pipe is called the Kelly. Each time 30 feet more the well is drilled, the connection at the bottom of the Kelly is unscrewed and a joint of drill pipe is added to allow further drilling. This causes high wear and reduced life on the relatively expensive Kelly. A short inexpensive section of pipe is normally added to the bottom of the Kelly to take this wear and is typically called a Kelly Saver.
Prior art valves characteristically do not allow the bore thru the valve to be opened for service access down into the string of drill pipe and then put back into service without having to disassemble the mud saver valve from the drill string to put it back together. Some of the alternatives, i.e. U.S. Pat. Nos. 3,698,411 and 3,965,980 require breaking a cap portion at the top of the valve to allow such service. U.S. Pat. No. 4,128,108 discloses a mud saver valve which requires that a pin be sheared to allow such service. U.S. Pat. No. 3,331,385 discloses a valve in which an extra part is added with special running tools to allow opening and then plugging the bore. This provides the limitations of making the hole available for servicing smaller, adding extra parts, and not allowing the critical wear surfaces to be retrieved easily for inspection and/or replacement.
A second problem associated with prior art valves is that of allowing any pressure build-up below the mud saver valve to be sensed by pressure gages above the mud saver valve. This might occur when the mud is not being pumped. If unstable well conditions exist in which a blowout threatens, watching the pressure in the drill pipe above this valve is critical in the process of knowing how to control the well. Typical prior art valves such as those listed above included added components to act as check valves to allow flow in the direction going up the well.
Prior art valve U.S. Pat. No. 4,899,837 provides a similar construction the present invention, however, when flow is reduced, it will prematurely start to close adding throttling wear to the internal components.